In some electro-statographic printing techniques, the printing process begins with placing a uniform electrostatic charge on a photoreceptor and exposing the photoreceptor to a light and shadow image or to a scanning laser to dissipate the charge on the areas of the photoreceptor exposed to the light to form a latent electrostatic image. The resultant latent image is developed by subjecting the latent image to a liquid toner comprising a carrier liquid and colored toner particles. These toner particles are generally comprised of a pigmented polymer. Generally, the development is carried out, at least partially, in the presence of an electric field, such that the toner particles are attracted either to the charged or discharged areas, depending on the charge of the particles and the direction and magnitude of the field.
The developed image may then be transferred to a substrate such as paper or plastic film, often via an intermediate transfer member (“ITM”) which is typically covered with a replaceable print blanket. The transferred image may then be permanently affixed to the substrate by the application of pressure, heat, solvent, overcoating treatment or other affixing processes. In general, in the commercial process used by HP-Indigo, the ITM is heated to a temperature that causes the toner particles and residual carrier liquid to form a film in the printed areas which film is transferred to the final substrate by heat and pressure. Fixing to the final substrate takes place as part of the transfer process.
The use of ITMs, and ITMs including transfer blankets, is well known. Multi-layered intermediate transfer print blankets for toner imaging are also known in the art. Generally, such blankets include a thin, multi-layered, silicone-based image transfer layer and a base (or body) portion which supports the image transfer layer and provides the print blanket with resilience during contact with an imaging surface and/or a final substrate. U.S. Pat. No. 5,745,829 to Gazit, et al. and U.S. Pat. No. 6,551,716 to Landa et al., the disclosures of which are incorporated herein by reference, describe print blankets for use with an ITM.
Other methods of manufacturing intermediate transfer members in the form of blankets and other types of blanket substrates are described, for example in U.S. Pat. No. 5,089,856 or U.S. Pat. No. 5,047,808, the disclosures of which are incorporated herein by reference.
In general, most ITM transfer surfaces are formed of a layer of silicone or a silicone rubber. For heated intermediate transfer members the surface should be abhesive to the hot toner material over repeated cycles of transfer to, heating on and transfer from the transfer surface. In general, silicone materials are superior to other materials in these properties and are used as transfer layers for ITMs for this purpose. Since various types of silicone materials are more or less abhesive to the hot toner (which is a hot melt adhesive) and have varying degrees of ruggedness, softness and other required mechanical properties, the choice of a material for transfer layers is a complex task.
A disadvantage of silicone layers, which is directly related to the liquid absorption, is a problem called “image memory”. This problem is believed to be caused by uneven absorption of carrier liquid over the surface of the blanket. It is understood that the amount of carrier liquid that is absorbed at different portions of the surface depends on whether these portions have toner particles or not. If a next following color separation has a different distribution of toner, then the next image may, under some circumstances have varying values of gloss or even varying amounts of toner transfer depending on the amount of liquid absorbed from the previous layer.
Another disadvantage of silicone based image transfer layers pertains to their useful lifespan. Repetitive swelling and drying of the print blanket, and specifically the image transfer layer, often results in the degradation of the mechanical properties of the print blanket. Over time, this expansion and contraction of the print blanket, due to swelling, necessitates the replacement of the print blanket which can be time consuming and costly.
Nevertheless, despite the inherent problems associated with silicones as release layers, they are the release material of choice for liquid toner ITMs.
Fluorinated Teflon® and other similar materials have been used in offset printing blankets, see EP 0 629 514 which is incorporated herein by reference. However these materials are generally considered not suitable for liquid electro-statographic printing due in part to manufacturing considerations and the resultant properties of the Teflon® based blanket after manufacture. For example, in the thickness that fluorinated Teflon® can be reliably and continuously laid down during manufacture, the overall blanket is too hard for suitable use in liquid electro-statographic printing.